I found a new web site that has replacement keys for Mono/Polys
(same as Poly-61s and PolySix's) as well as knobs, semiconductors,
and miscellaneous parts that vary depending on what's available. I
had the hardest time finding it using Google, so I tried search.com,
searched for "Mono-Poly", and used the "search within these results"
feature to look for "keys". (Looking for "replacement keys" using
this strategy didn't work.) Apparently I was his first customer; he
gave me a nice discount on the keys I've been looking for since I
posted my first message months ago, and he threw in the springs at
no charge. He accepts PayPal as well as the other usual payment
options. The web link is:

http://cu.imt.net/~sounddoctorin/synthtec/korg/monopoly.htm

He also has parts in stock for various other analog synths, is very
well versed in the technologies of monophonic synths, and has
several vintage analog synths for sale including a Mono-Poly! I
hope this information is useful for those of you who have been
combing ebay for replacement knobs and keys.

Good luck!

Joe